Power bipolar transistors are widely used as switches in static power conversion circuits. In many of these applications it is desirable to have switch turn-on and turn-off times as short as possible. One component of the turn-off time is the storage time. This is the time required to remove the minority charge injected into the base region that is in excess of what is needed to make the transistor saturate. To reduce the storage time in a conventional low voltage bipolar transistor, the standard solution is to connect a Schottky diode between the base and collector electrodes of the transistor. This diode reduces the amount of minority charge injected into the base region as the transistor approaches saturation. The transistor is thus prevented from becoming saturated because some of the base drive current is diverted through the Schottky diode. (See, e.g., Herbert Taub et al., "Digital Integrated Electronics", McGraw Hill, Inc., 1987, pp. 49-51).
In power bipolar transistors, however, simply connecting a Schottky diode between base and collector electrodes is not suitable for reducing the minority charge injected into the base. One reason is that there is a substantial voltage drop across the bulk resistance of the collector region when the transistor is conducting. Another reason is that the collector-emitter voltage V.sub.CE is not well defined in saturation and is a function of the collector current. Moreover, the collector-base P-N junction in a power bipolar transistor extends over a greater distance in the chip than it does in a signal or low power bipolar transistor, and connecting a Schottky diode between the base and collector electrodes does not provide a base drive current diversion path that is distributed across the collector-base junction. As a result, performance of the Schottky diode is dependent solely on the transistor electrode voltages, and the diode conducts increasing amounts of current as collector-base voltage increases. Lead inductance therefore becomes a factor, and the Schottky diode effectiveness in preventing saturation and hence reducing storage time can actually be diminished if this inductance results in increased voltages on the transistor leads. On the other hand, if the Schottky diode can be integrated with the transistor by intimately contacting the collector region over a wide area thereof, the diode can thus be made sensitive to collector-base junction voltage and can divert base drive current with uniform distribution over the entire collector-base junction. In addition, inductive voltages in the transistor leads would have no effect on the Schottky diode thus integrated with the transistor.
Accordingly, one object of the present invention is to provide a high power bipolar transistor device having a short turn-off time.
Another object of the present invention is to provide a high power bipolar transistor device in which excess minority charge buildup in the base region is avoided.
Another object of the present invention is to provide a high power bipolar transistor device with integral diode means for limiting forward bias on the collector-base junction thereof.
Excess base drive current affects the operation of other semiconductor devices, such as thyristors and insulated gate transistors (IGTs), which incorporate bipolar transistor structures. Specifically, a thyristor is a semiconductor device comprising four contiguous layers of successively alternate conductivity type. The device can be switched from a non-conducting state to a conducting state under control of a gate. A detailed description of thyristor devices can be found, for example, in S. K. Ghandi, "Semiconductor Power Devices", pp. 188-243 (John Wiley & Sons, 1977). A thyristor can be considered analogous to a PNP and an NPN bipolar transistor, connected such that the base of each is driven by the collector current of the other. Once the thyristor is turned on via a gate electrode which supplies current to the base of either of the two transistors comprising the thyristor, the transistors drive each other into saturation if the individual transistor gains are sufficiently large. When this happens, the thyristor is no longer under control of its gate electrode and continues to conduct even in the absence of gate drive current. This phenomenon is known as regenerative latch-up. Accordingly, a further object of the present invention is to provide a thyristor which includes integral diode means for limiting thyristor latch-up.
An IGT is a device which combines a metal oxide semiconductor (MOS) gate structure with bipolar current conduction. The IGT exhibits high forward conduction current density, low drive power to an MOS gate structure, fully gate controlled output characteristics with gate turn-off capability, and a unique reverse blocking capability. However, the IGT includes an inherent parasitic P-N-P-N thyristor structure. If this parasitic thyristor latches in the manner described above, current through the IGT can no longer be controlled by the gate. A discussion of IGT devices including prior art techniques for preventing latch-up of the parasitic thyristor can be found in B. J. Baliga, "Modern Power Devices", John Wiley & Sons, 1987, pp. 350-401. A further object of the present invention, therefore, is to provide an IGT with integral diode means for preventing latch-up of the inherent parasitic thyristor.